The present invention relates generally to infrared (IR) cameras and detectors. More particularly, the present invention relates to a lens system and method for use with a lightweight infrared (IR) camera.
Infrared cameras and sensors in general, and microbolometer cameras in particular, are well known to those skilled in the art. See, for example, U.S. Pat. Nos. 5,688,699; 5,999,211; 5,420,419; and 6,026,337, all of which are incorporated herein by reference. Infrared microbolometer cameras typically include an array of infrared sensitive sensing detectors, each having a resistance that changes with temperature, with each detector having an infrared absorber that may be formed in several ways. See, for example, U.S. Pat. Nos. 5,939,971 and 5,729,019, herein incorporated by reference.
During operation, the incoming infrared radiation heats each sensing detector in proportion to the amount of infrared radiation received. The sensing detectors are then queried, typically one by one, to determine the resistance of the sensing detectors, and thus the amount of infrared radiation received. Typically, supporting electronics are provided with the camera to process the detector output signals, provide calibration and compensation, and provide a resulting image.
Because heat is used to measure the amount of incoming infrared energy, changes in the ambient temperature of the microbolometer array can significantly affect the detector signals. To compensate for this, many infrared cameras or detectors have a thermoelectric stabilizer to regulate the temperature of the array. In one example, thermoelectric stabilizers are used to maintain the array temperature at a known value. A limitation of using thermoelectric stabilizers is that they can draw significant power and can add significant weight to the system.
Because of manufacturing tolerances, each sensing detector in the camera or detector may have a slightly different zero point than other detectors within the system. To compensate for these detector-to-detector differences, many infrared cameras or detectors have a means for providing a zero radiation baseline value, which is made available to interpret or calibrate the detector output signals. One method for providing the zero radiation baseline is to use a shutter or chopper to periodically block the incoming infrared energy. When the shutter or chopper is activated, a zero radiation baseline is read and stored. A limitation of this approach is that the shutter or chopper can add significant complexity and weight to the system, which for some applications, can be particularly problematic. Another approach for providing a zero radiation baseline is to periodically point the camera at a uniform infrared source such as the sky. This, however, can require significant control circuitry to periodically change the direction of the camera, again adding weight to the system.
Typically, a lens system is used to focus the incoming infrared radiation on the sensing detectors. The lens system often includes a triplet of lenses. The first lens typically forms an image having significant distortion, such as spherical aberration. The focal plane of the first lens often is a focal surface which is curved, and produces a distorted image if provided directly to a planar detector array. The distorted image is often corrected by one or more correcting lenses. The correcting lenses can be used to produce a useable image, but add significant weight to the infrared camera.
For some applications, the weight of the infrared camera can be important. For example, for lightweight micro air vehicle (MAV) applications, the weight of the infrared camera can impact the size, range and other critical performance parameters of the vehicle. As such, a lightweight infrared camera having a lightweight lens system would be highly desirable.